1. Field of the Invention
The present invention is related to a technique of compacting a high-frequency-purpos transformer by reducing a copper loss (load loss) and by improving electromagnetic coupling of windings (coils).
2. Description of the Related Art
An ignition circuit for discharge lamps such as a metal halide lamp is generally equipped with a DC-to-DC converting circuit, a DC-to-AC converting circuit, and a starting circuit. A pulse-width modulation (PWM) system and a pulse-frequency modulation (PFM) system are used as a control system for switching power supply circuit which constitutes a DC-to-DC converting circuit (DC-to-DC converter).
When a flyback type structure is used as a DC-to-DC converting circuit, a converting transformer (converter transformer) is required, and a construction suitable for a high-frequency switching control operation is required in order to make this converter transformer compact.
When circular wires are employed as windings (coils), a skin effect caused by high-frequency currents may present a problem. That is, copper losses can increased and electromagnetic coupling conditions can degrade with the circular wire windings.
This skin effect may effectively reduce a sectional area for current flow when a high-frequency current flows through a conductor because the high-frequency current may be restricted to flow only in a certain limited area of a conductor surface. For a circular wire, copper losses may increase because an effective volume of high-frequency current may not be sufficiently secured as compared to a volume of a winding of this circular wire.
A transformer made of an alternately-overlapping arrangement (so-called “sandwich winding”) has respective coils (windings) that are sequentially wound with respect to a cylindrical portion which constitutes a coil bobbin. To use this transformer in a high frequency field, a total turn number of the coils must be small in order to reduce the inductance. If a circular type wire is used in such a transformer, an electromagnetic coupling characteristic between a primary winding (primary coil) and a secondary winding (secondary coil) would deteriorate because of air gaps between the sandwich windings.